If the given Either is a Right and projected to a Left, the Left operations have no effect on the Right value.
If the given Either is a Left and projected to a Right, the Right operations have no effect on the Left value.
If a Left is projected to a Left or a Right is projected to a Right, the operations have an effect.

Example: A compute() function, which results either in an Integer value (in the case of success) or
in an error message of type String (in the case of failure). By convention the success case is Right and the failure
is Left.

peek

Performs the given action on the first element if this is an eager implementation.
Performs the given action on all elements (the first immediately, successive deferred),
if this is a lazy implementation.